This invention concerns the grinding of workpieces such as crankpins and the cam regions of cam shafts, where the grinding wheel performing the grinding is moved towards and away from the axis about which the workpiece is rotating so as to maintain engagement with the surface thereof which is to be ground, as the workpiece rotates around its main axis such as in the case of a crankpin which precesses around the main crankshaft axis, as the latter rotates.
The advance and withdrawal of the grinding wheel is normally under computer control and with the current development of grinding machines, errors which hitherto were present in ground workpieces have been largely eliminated by appropriate programming and secondary errors which were previously masked by the larger process errors, have now begun to be revealed.
Errors such as out of roundness of 1 or 2 microns, can result in unwelcome wear of a final component such as between a crankpin and lower big end bearing.
Errors which have already been accommodated, can arise from the varying height of the axis of the workpiece region which is being ground (such as the orbital movement of a crankpin as the crankshaft rotates), relative to the horizontal plane containing the axis about which the grinding wheel rotates. Typically the throw of a crankshaft is the order of a few centimeters and there is thus a considerable variation in height of the axis of the pin relative to the horizontal plane containing the wheel axis of rotation as the pins are rotated due to the rotation of the crankshaft. The grinding wheel is moved towards and away from the crankshaft so as to maintain the grinding contact with the surface of the pin at all times as the latter is rotated around the main crankshaft axis, but, assuming that the crankpin axis lies in the same horizontal plane as the axis of rotation of the grinding wheel, there are only two points during each rotation of the crankshaft when the pin axis also occupies that same plane. These are at 3 o""clock and 9 o""clock positions. At the 12 o""clock and 6 o""clock positions, the pin axis will be at the maximum displacement above and below the plane and at all intermediate positions, the height of the pin will vary relative to the plane.
The reference to a horizontal plane presupposes that the movement of the grinding wheel is in a horizontal sense without any divergence therefrom. This is normally the case but for the avoidance of doubt, it is to be understood that if the locus of the grinding wheel axis as the latter is moved towards and away from the workpiece, is in a plane which is not horizontal, the same considerations still apply with regard to the alignment of the crankshaft axis with the wheel axis, except that the xe2x80x9c3 o""clockxe2x80x9d and xe2x80x9c9 o""clockxe2x80x9d positions now correspond to when the crankpin axis lies within the plane containing the path of the movement of the wheel axis.
Computer controlled grinding machines have been programmed to alter the wheelhead demand positions during the crankpin rotation, to compensate for the errors which can result from the varying height of the crankpin as the crankshaft rotates. Such a machine will be referred to as xe2x80x9cof the type describedxe2x80x9d.
In the more general case, the main axis of rotation of the crankshaft (or cam shaft as the case may be) will not normally occupy the same plane as the path of movement of the grinding wheel axis as the latter is moved towards and away from the workpiece, so that there is a constant height error to be taken into account. Effectively this introduces a degree of non-symmetry into the errors arising during the rotation of the crankshaft or cam shaft, which would generally be symmetrical if the workpiece axis and grinding wheel axis occupied the same plane as the path of movement of the grinding wheel axis towards and away from the workpiece.
It is an object of the present invention to provide a solution to this problem.
According to the present invention in a computer controlled grinding machine programmed so as to control the machine by calculating the wheelhead demand positions so as to grind the desired workpiece using appropriate parameters for the workpiece such as roundness, diameter, throw and taper (if required) based on the assumption that the workpiece axis and grinding wheel axis occupy the same plane as does the path of movement of the wheel axis towards and away from the workpiece, wherein the machine is also programmed to alter the wheelhead demand position during workpiece rotation to compensate for errors resulting from the varying height of the workpiece as the latter rotates, and wherein a demand position value is computed which takes into account the difference in height between the workpiece axis of rotation and the grinding wheel axis of rotation for each of a plurality of rotational positions of the workpiece around its axis and stored for each position, prior to grinding, and the wheelhead position demand signals employed during grinding of the workpiece are derived from the stored values.
If the difference in height between a crankshaft workpiece axis and the wheel axis is H, then in accordance with the invention, the demand position value (P) for each angular position of the workpiece A (measured in the direction of rotation of the workpiece around its main axis from a start position) is given by the following equation:
P=(T* cos A)+(R+r)2xe2x88x92((T* sin A)+H)2)xe2x80x83xe2x80x83(1) 
Where:
R is the current radius of the grinding wheel,
r is the target radius for the crankpin, and
T is the throw of the crankpin around the main crankshaft axis.
Typically the grinding wheel rotates in one sense, e.g. clockwise, and the crankshaft rotates in the opposite sense, e.g. anti-clockwise, and the start position is when the grinding wheel is at its furthest (most rearward) position relative to the crankshaft axis, and the crankpin and crankshaft axes occupy the same horizontal plane.
Typically the computed value for P is calculated for each of 3600 positions during one revolution of the workpiece, ie from A=0 to 2xcfx80 (which in the case of a rotating crankshaft results in turn in one revolution of the crankpin about its axis).
Preferably during grinding of the crankpin, the value for P is calculated at each of a succession of equally spaced apart points in time from the beginning of the grind, by using the appropriate value for P from the stored values of P, or where the angular position of the workpiece at any instant does not correspond precisely with an angular position at which a value for P has been stored, a value for P is computed by interpolating between the two adjoining stored values for P.
It has been found that a 0.1 millimeter height discrepancy H can result in a 1 micron roundness error, ie a 1 micron necking of what should otherwise be a circular cross-section.
The invention also lies in a computer controlled grinding machine as aforesaid in which the computer is loaded with a program and operated to calculate and store in a memory the demand position (P) for the wheelhead using and equation for (P) taking account of any non-circularity or non-concentric rotation of the workpiece, together with any difference in height between the workpiece and wheel axes, for each of a plurality of positions during one revolution of the workpiece, and the wheelhead feed is subsequently controlled by signals derived from the stored values of (P), during a subsequent grinding of the workpiece.
The invention also lies in a method of controlling the wheelhead of a computer controlled grinding machine so as to accommodate errors which would arise due to misalignment of the horizontal planes containing the wheel axis and the main axis about which the workpiece is rotated; wherein as a first step, a computer is loaded with a program which enables the instantaneous demand position for the wheelhead (P) to be calculated for each of N positions of the workpiece for a single revolution of the workpiece, and storing the computed value of (P), and as a second step, during grinding of the workpiece, computing the demand position for the wheelhead at each of a succession of equally spaced apart points in time from the start of grinding, by relating the time to the angular position of the workpiece and using the N stored values and interpolating between them where values for P required are intermediate the values stored for particular angular positions, and as a third step generating a demand position control signal for controlling the wheelhead during grinding using the stored and/or interpolated demand position values for P.
Preferably in the above method the value of P is recalculated at 1 ms intervals during the grinding.
The invention also lies in workpieces when ground using a grinding machine as aforesaid or a grinding machine operating in accordance with the above method.